The present invention relates to an objective lens used in an optical pickup for recording and/or reproducing signals of a mini disc (MD), magneto-optical disc (MO), compact disc (CD), CD-ROM or the like (hereinafter referred to as "optical disc"), and also to an optical pickup and an optical disc apparatus equipped with such an objective lens.
According to the related art, an exemplary optical pickup designed for an optical disc has the structure shown in FIG. 6.
In FIG. 6, the optical pickup 1 comprises a semiconductor laser element 2, a grating 3, a beam splitter 4, an objective lens 5 and an optical detector 6.
The grating 3 is a diffraction type which divides an incident light beam emitted from the semiconductor laser element 2 into a main beam of zeroth-order light and side beams of plus and minus first-order light.
The beam splitter 4 is so disposed that its reflecting surface has an inclination of 45.degree. to the optical axis, and separates the return light, which is obtained from the signal recording plane of the optical disc D, from the light beam emitted from the semiconductor laser element 2. More specifically, the light beam emitted from the semiconductor laser element 2 is reflected by the reflecting surface 4a of the beam splitter 4, while the return light obtained from the optical disc D is transmitted through the beam splitter 4.
The objective lens 5 is convex and forms the light beam reflected by the beam splitter 4 into an image on a desired track on the signal recording plane of the optical disc D which is driven to be rotated. Further the objective lens 5 is supported by an unshown biaxial actuator, i.e., to be movable biaxially in a focusing direction and a tracking direction.
The optical detector 6 has a light receiving portion to receive the return light beam which is incident thereon after transmission through the beam splitter 4.
According to the optical pickup 1 of the structure described above, the light beam emitted from the semiconductor laser element 2 is divided by the grating 3 into a main beam and two side beams and, after being reflected by the reflecting surface 4a of the beam splitter 4, the beams are formed, via the objective lens 5, into an image at a certain point on the signal recording plane of the optical disc D.
The return light beam reflected by the signal recording plane of the optical disc D is incident on the beam splitter 4 via the objective lens 5 again. Here, the return light beam is transmitted through the beam splitter 4 and then is incident on the light receiving portion of the optical detector 6.
Thereafter, the information recorded on the signal recording plane of the optical disc D is reproduced on the basis of a detection signal outputted from the light receiving portion of the optical detector 5, and simultaneously a focus error signal FE and a tracking error signal TE are detected.
For accurate detection of the reproduced signal, the light beam emitted from the semiconductor laser element 2 is formed into a spot at a correct position on the signal recording plane of the optical disc, and the objective lens 5 is finely moved by an unshown biaxial actuator in accordance with the focus error signal FE and the tracking error signal TE, so as to perform exact reproduction of the recorded signal.
In the optical pickup 1 of the above structure, the objective lens 5 is generally composed of a plastic or glass material and is shaped into a single optical lens with aspherical surfaces. And, as shown in FIGS. 7A and 7B, an iris diaphragm 7 is disposed adjacently to the objective lens 5 so as to regulate the NA (numerical aperture) of the light beam irradiated onto the signal recording plane of the optical disc D via the objective lens 5.
FIG. 7A shows an exemplary structure where an iris diaphragm 7 for regulating the NA (numerical aperture) of the light beam consists of an independent member separate from the objective lens 5 and is disposed adjacently to the lens 5, and FIG. 7B shows another structure where an iris diaphragm 7 is formed in a lens holder 8 which holds an objective lens 5.
Consequently, the light beam to be incident outside the optical effective surface of the objective lens 5 is partially intercepted by the iris diaphragm 7 as illustrated in FIG. 7, whereby any unnecessary partial light beam is eliminated to prevent any harmful influence when recording the information on or reproducing the same from the optical disc.
However, since the iris diaphragm 7 shown in each of FIGS. 7A and 7B is an independent component member separate from the objective lens 5, the total number of required component members is thereby increased, and in assembling the apparatus, it is necessary to align its optical axis with the objective lens 5 which to eventually brings about a problem of raising the costs of both component members and assembly.
And there exists another problem that, when the iris diaphragm 7 fails to be exactly aligned with the objective lens 5, asymmetry aberration is generated due to the surface shape outside of the optical effective surface of the objective lens 5.
There is proposed another known art wherein the surface shape of the objective lens 5 on the light source side is modified, as shown in FIG. 9, instead of using the iris diaphragm 7 mentioned above, and a recess 5a is formed outside the optical effective surface so as to give an iris diaphragm function to the objective lens 5 itself.
According to this method, the light beam to be incident outside the optical effective surface is actually incident on the recess 5a of the objective lens 5, so that such light beam is prevented from reaching the spot position on the signal recording plane of the optical disc D. Therefore, regarding the spot position on the signal recording plane of the optical disc D, the partial light beam incident outside the optical effective surface is intercepted to consequently achieve the same function as the iris diaphragm 7.
However, in the objective lens 5 with such recess 5a, it is difficult, when molding the same out of plastics or the like, to maintain an exact surface shape due to post-molding contraction and so forth if there is any sharp contour change such as a stepped region 5b where the curved surface is rendered discontinuous inside the recess 5a. For example, at a corner denoted by reference symbol A in FIG. 9, spherical aberration is generated due to the distortion caused in the molding process, hence raising a problem that the optical characteristic of the objective lens 5 is deteriorated.
There is further proposed another example of FIG. 10, wherein two spots are formed along an optical axis by a single objective lens 5, so as to reproduce signals from a plurality of optical discs of mutually different standards. However, in execution of focusing control with such objective lens 5 that forms two spots, an S-signal of the spot not in use may be detected as a false signal to eventually raise a problem that two S-signals are generated. For this reason, it is considered difficult to perform exact detection of the focus error.